1. Field of the Invention
The present invention relates to a battery pack. In particular, the present invention relates to a battery pack that has increased attachment strength of a protective circuit module that is molded by a resin and is positioned on top of a battery that comprises an electrode assembly, a can, and a cap assembly.
2. Description of the Prior Art
Recently, rechargeable batteries have been widely researched and developed because they can be manufactured in a compact size and can provide significant amounts of voltage. Typical examples of rechargeable batteries that have recently been developed include nickel-hydrogen (Ni-MH) batteries, lithium (Li) batteries, and lithium ion batteries.
Generally, the battery (rechargeable or nonrechargeable) of a battery pack is formed by placing an electrode assembly comprising positive and negative electrode plates and a separator into a can that is made of aluminum or an aluminum alloy. The can is topped with a cap assembly, an electrolyte is injected into the can, and the can is sealed. When the can is made of aluminum or an aluminum alloy, the battery is lighter due to the low weight of aluminum and does not corrode even when it is used for a long period of time under a high voltage.
The battery normally has an electrode terminal that is positioned on top of the battery while being insulated from the periphery. The electrode terminal is connected to an electrode inside the battery and acts as either a positive electrode or a negative electrode. The can itself has the opposite polarity of the electrode terminal.
Under certain circumstances, a battery may discharge a large amount of energy at one time. In particular, a charged battery pack stores energy that has been supplied from another energy source during a charging process. When a malfunction such as an internal short circuit occurs, the energy that has accumulated in the battery is discharged quickly and may result in a fire or explosion.
In the widely-used lithium-based rechargeable batteries, the lithium itself has a high reactivity and is very likely to catch on fire or to explode when the battery malfunctions. In lithium ion batteries, lithium exists not in a metal state but in a much safer ionic state. However, the materials that are used as the negative electrode and the non-aqueous electrolyte are flammable and are very likely to catch fire or to explode when the battery malfunctions.
For these reasons, a battery pack is provided with various safety devices to prevent it from catching fire or exploding when it has been charged or while it is being charged. Such safety devices interrupt current flow when the temperature or voltage of the battery rises due to overcharging or over-discharging. The safety devices include a protective circuit module that is connected to the battery and is adapted to sense abnormal current or voltage and interrupt the flow of current, a positive temperature coefficient (PTC) thermistor that is actuated by overheating due to abnormal current, and a bimetal.
It is generally difficult to couple the electrode of a battery to the electrical terminal of a protective circuit module by direct welding because of the shape and composition of the battery. Therefore, a conductive structure referred to as a “lead plate” is used to couple the positive electrode and negative electrode of the battery to the electrical terminal of the safety device. The lead plate is usually made of nickel, nickel alloy, or nickel-plated stainless steel.
FIG. 1 is an exploded perspective view of a conventional lithium ion battery pack before it is coupled by a molded resin. FIG. 2 is a perspective view of a battery pack that has a protective circuit module that is molded from a molded resin and is attached to a battery.
Referring to FIG. 1 and FIG. 2, a can-type battery pack includes a battery 10 and a protective circuit module 20. The battery 10 includes a can 12 which contains an electrode assembly and is sealed on top by a cap plate 13.
The cap plate 13 has a size and a shape corresponding to the open top of the can 12. The cap plate 13 has a negative electrode terminal 14 that is formed at its center, as well as a negative electrode connection lead 16 and a lead plate 18 that are coupled to both sides of the top surface of the cap plate 13. An end of the negative electrode connection lead 16 is coupled to the negative electrode terminal 14 and a surface of the other end protrudes to the top surface of the cap plate 13. A surface of the lead plate 18 protrudes to the top surface of the cap plate 13.
Reference numeral 15 in FIG. 1 refers to an insulation plate that is positioned in between the negative electrode connection lead 16 and the cap plate 13.
The protective circuit module 20 includes a top surface that has external input and output terminals 22 and 23, respectively that are formed thereon. The bottom surface of the protective circuit module 20 has a circuit portion (not labeled) and connection terminals 26 and 28 that are formed thereon. The negative electrode connection terminal 26 and a positive electrode connection terminal 28 and are L-shaped so that they can be coupled to the negative electrode connection lead 16 and the lead plate 18 of the cap plate 13, respectively. The negative electrode connection lead 16 and the lead plate 18 of the cap plate 13 are generally coupled to the connection plates 26 and 28 by resistance spot welding.
The protective circuit module 20, as shown in FIG. 2, is molded by a molded portion 30 and is attached to the top of the battery. The external input and output terminals 22 and 23 are exposed to the top surface of the molded portion 30.
The protective circuit module 20 is fixed to the top of the battery 10 by forming a molded portion 30 from a resin. However, the cap plate 13 and the lead plate 18 that contacts the molded portion 30 are made of metal and the contact area is typically small. Consequently, the attachment strength of the protective circuit module 20 to the battery 10 is weak.
When the molded portion is subjected to a force in the lateral direction or when the bending external force that acts on the contact surface of the molded portion becomes larger, the molded portion may be easily twisted from the battery.